Compositional analysis and functional insights of the cell envelope in Synechocystis sp. PCC 6803

The cell envelope of Synechocystis sp. PCC 6803 has a Gram-negative-like organization, with inner and outer membranes and an outermost surface layer. A comprehensive characterization of its layered structure, considering the presence of pigments and distinctive proteins, has not yet been established. Here, we isolated cell envelope fragments and characterized them by electrophoretic and proteomic analyses identifying a recurring set of inner membrane, outer membrane, and S-layer proteins, including six conserved porins and additional associated proteins with repeated beta-strand motifs, among which several contain a SLH domain. Further characterization by absorption spectroscopy, to investigate the typical orange color of this cell envelope, revealed extended retention of carotenoids. These results provide an integrated and preliminary view of the Synechocystis sp. PCC 6803 cell envelope and its characteristic proteome.